Fast photodetectors are required for many applications, including optical interconnects and optical communications. In many cases it would be highly desirable to have available a monolithically integrated photodetector/receiver combination. Such a combination has been demonstrated by N. Yamanaka et at. (IEEE Journal on Selected Areas in Communications, Vol. 9(5), p. 689). The GaAs-based combination exhibited excellent performance using laser transmitters operating at 0.85 .mu.m wavelength.
Those skilled in the art will recognize that a Si-based photodetector/receiver combination would be substantially less expensive than the prior art GaAs-based combination. Furthermore, more electronic functionality could typically be built into a Si-based combination than into a GaAs-based combination, due to the far greater maturity of Si technology. Unfortunately, Si has an indirect bandgap and thus has much weaker absorption for electromagnetic radiation than GaAs. For instance, radiation of wavelength .lambda.=0.85 .mu.m and 0.88 .mu.m, respectively, has an absorption length .alpha..sup.-1 =15 .mu.m and 20 .mu.m in Si, compared to a respective absorption length of about 1 .mu.m in GaAs.
The long absorption length of electromagnetic radiation in crystalline Si produces two problems. First, since the depletion width (and thus the carrier collection length) in a Si-based photodetector (e.g., a metal-semiconductor-metal detector) will only be a few microns, the quantum efficiency .eta. of the detector will be quite low. Second, the photocarrier generated below the depletion region will be collected by diffusion rather than field induced drift, resulting in a slow detector. Thus, prior art Si-based photodetectors typically can not simultaneously have high (e.g.,.gtoreq.20%) quantum efficiency and high (e.g.,.gtoreq.1 Gb/s) speed. For instance, a prior art Si MSM (metal-semiconductor-metal) detector, comprising a 0.51 .mu.m thick crystalline Si layer on sapphire, had only 0.6% efficiency at .lambda.=0.85 .mu.m, with a response to pulse excitation that had full width at half maximum (FWHM) of 5.7 ps for red light excitation. See C.-C. Wang et at., Applied Physics Letters, Vol. 64 (26), p. 3578. Clearly, the prior art detector had high speed but very low efficiency, and thus would not have been suitable for many potential applications. M. Y. Liu et al., Applied Physics LettersVol. 65(7), p. 887 (1994) suggest that the responsivity of a SMS Si-based photodetector can be improved, without lowering of device speed, by means of a quarter-wave stack reflector underneath the active layer, and an antireflection coating on the active layer. These suggested modifications would result in a relatively complex, and therefore costly, structure.
In view of the potential advantages of a Si-based photodetector, it would be highly desirable to have available a Si-based photodetector of simple structure that can have both relatively high quantum efficiency and fast response. This application discloses such a photodetector.
Glossary
By a "textured" surface we mean herein a surface that exhibits surface features, typically substantially randomly distributed features, so as to (randomly or pseudo-randomly) scatter radiation of a predetermined wavelength .lambda. incident on the surface. Associated with such a surface are statistical measures of the surface features, including an average feature size. The average feature size typically can be determined from, e.g., a scanning electron microscope (SEM) micrograph of the surface.